This invention relates in general to spectrometers and, in particular, a refractive-diffractive spectrometer.
Spectrometers or spectrophotometers are widely used devices. They are used in digital printers and printing presses. Hand-held spectrometers are used by graphic designers and imaging departments at newspapers, magazines, and copy shops.
In order to analyze the spectro data from images or objects, light from the images or objects is passed through optical elements to a detector, such as a charged coupled device (xe2x80x9cCCDxe2x80x9d) array. In order to accurately measure the CIE tristimulus values of light sources, images or objects, it would be desirable to accurately resolve all of the wavelength components in the radiation from the light source. Spectrometers resolve such wavelength components by dispersing them at different angles depending on the wavelength. Unfortunately, up to the present time, spectrometers and spectrophotometers do not disperse the different wavelengths linearly. This means that after being dispersed by the spectrometer into the different wavelength components reaching the CCD array, the dispersion of a particular wavelength component is not proportional to the wavelength of the component. For example, if a prism is used in the spectrometer for dispersing the wavelength components of radiation from a source, the angle of refraction of any wavelength component is not proportional to its wavelength.
Radiation from many light sources can have a large number of spectral lines or wavelength components. Therefore, unless the CCD array has the same number of detectors as the number of wavelength components, at least some of the spectral lines or wavelength components of the light source will be directed to positions along the CCD array that does not fall entirely on any particular detector, but may fall partly on one detector and partly on another detector. Since the dispersion of the wavelength components is nonlinear, it cannot be assumed that a linear interpolation of the outputs of the two detectors will yield an accurate measurement of the intensities of such wavelength components. This causes error in measurement. Therefore, to accurately measure the CIE tristimulus values of light sources of filters that have fine spectral detail, spectrometers of the conventional design require higher spectral resolution. However, high quality spectrometers are expensive.
It is therefore desirable to provide improved spectrometers and spectrophotometers in which the above-described disadvantages are avoided.
This invention is based on the recognition that, by employing two optical elements having a combined dispersive characteristic such that they substantially linearly disperse electromagnetic radiation over at least a portion of an electromagnetic spectrum, the above-described disadvantages of conventional spectrometers and spectrophotometers can be avoided. In the preferred embodiment, a refractive element such as a prism and a diffraction grating may be employed. Preferably, the two optical elements are arranged so that the spectrometer is substantially telecentric; in such event, the spectrometer provides substantially the same magnification at different wavelengths in the spectrum. In other words, radiation energy will be dispersed also linearly across the portion of the electromagnetic spectrum.
Where the combined dispersive characteristic of the two elements is substantially linear, linear interpolation of the type described above would not introduce significant interpolation errors, in contrast to the conventional design of spectrometers. Therefore, even if high resolution CCD arrays are not used, radiation sources, images and objects having a rich spectrum can still be accurately measured. This drastically reduces the cost of the spectrometer.
Instead of actually detecting the different wavelength components, the wavelength components may be directed towards different optical channels in a demultiplexing arrangement. First, the light or radiation source may be an input optical channel carrying radiation of different wavelength components. After passing such wavelength components through the two optical elements, the wavelength components are dispersed substantially linearly. Therefore, irrespective of the wavelengths of the wavelength components in the input optical channel, one can be certain that a particular output channel is carrying a corresponding particular wavelength component. This is not possible if the combined dispersive characteristics of the two optical elements are not substantially linear. The above demultiplexing arrangement is bidirectional. In other words, the separate output channels in the above demultiplexing arrangement can instead become input channels. The wavelength components in such separate input channels, after passing through the two elements, will emerge as a combined beam towards the output channel (the input channel in the demultiplexing arrangement) in a multiplexer arrangement. Again, since the combined dispersive characteristic of the two elements is substantially linear, the different wavelength components will be combined into a single beam by the two elements irrespective of the wavelengths of the different input wavelength components.